Probe heater remaining useful life determination

ABSTRACT

A probe system includes a heater and a control circuit. The heater includes a resistive heating element routed through the probe system. An operational voltage is provided to the resistive heating element to provide heating for the probe system. The control circuit is configured to provide the operational voltage and monitor a capacitance between the resistive heating element and a metallic sheath of the heater over time. The control circuit is further configured to determine a remaining useful life of the probe system based on the capacitance.

BACKGROUND

The present invention relates generally to probes, and in particular toa system and method for determining a remaining useful life of aircraftsensor probes.

Probes are utilized to determine characteristics of an environment. Inaircraft systems, for example, probes may be implemented on the externalportions of the aircraft to aide in determination of conditions such asairspeed, Mach number and flight direction, among others. Due to theharsh conditions of flight, ice may build-up on portions of the probe.To combat this, heaters are implemented within the probe to prevent theformation of ice that may impact proper functionality of the probe.

When probes break down, they need to be replaced, often prior to asubsequent takeoff. The heating element of a probe is often the mostlife-limited part. Therefore, probes need to be replaced as soon as theheating element breaks down. It is desirable to predict a remaininguseful life of the probe heating element in order to better predictmaintenance needs of the probe itself.

SUMMARY

A system for an aircraft includes a probe and a control circuit. Theprobe includes a heater that includes a resistive heating element routedthrough the probe. An operational voltage is provided to the resistiveheating element to provide heating for the probe. The control circuit isconfigured to provide the operational voltage and monitor a capacitancebetween the resistive heating element and a metallic sheath of theheater over time. The control circuit is further configured to determinea remaining useful life of the probe based on the capacitance.

A method for determining a remaining useful life of a probe thatincludes a heater, the method includes providing an operational voltageto a resistive heating element of the aircraft probe during a pluralityof flights of an aircraft to provide heat for the probe; determining, bya control circuit, a capacitance between a resistive heating element ofthe heater and a metallic sheath of the heater during the plurality offlights; and determining the remaining useful life of the probe basedupon the determined capacitance over time.

A probe system includes a heater and a control circuit. The heaterincludes a resistive heating element routed through the probe system. Anoperational voltage is provided to the resistive heating element toprovide heating for the probe system. The control circuit is configuredto provide the operational voltage and monitor a capacitance between theresistive heating element and a metallic sheath of the heater over time.The control circuit is further configured to determine a remaininguseful life of the probe system based on the capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an aircraft that includes a pluralityof probes.

FIG. 2 is a diagram of an aircraft probe that includes a heatingelement.

FIG. 3 is a diagram illustrating a heating element of an aircraft probe.

FIG. 4 is chart illustrating functions of a monitored characteristic ofa plurality of probe heating elements over time.

FIGS. 5A and 5B are charts illustrating monitored current based on a lowvoltage for a heating element over time.

FIG. 6 is a chart illustrating a resonant frequency over time for aheating element of an aircraft probe.

FIGS. 7A and 7B are thermal images of an aircraft probe.

FIG. 8 is a flowchart illustrating a method of determining a remaininguseful life of a probe based upon detected micro-fractures.

FIG. 9 is a flowchart illustrating a method of determining a remaininguseful life of a probe based upon a monitored current draw over time bya heating element of a probe.

FIG. 10 is a flowchart illustrating a method of determining a remaininguseful life of a probe based upon a determined capacitance of a heatingelement of a probe.

FIG. 11 is a flowchart illustrating a method of determining a remaininguseful life of a probe based upon current leakage of a heating elementof the probe.

FIG. 12 is a flowchart illustrating a method of determining a remaininguseful life of a probe based upon a resonant frequency of a heatingelement of the probe.

FIG. 13 is a flowchart illustrating a method of determining a remaininguseful life of a probe based upon thermal imaging of the probe.

FIG. 14 is a flowchart illustrating a method of determining a remaininguseful life of a probe based upon an antenna response of a heaterelement of the probe.

DETAILED DESCRIPTION

A system and method for determining the remaining useful life of a probeis disclosed herein that includes monitoring characteristics of theprobe over time. The probe, which may be an aircrafttotal-air-temperature (TAT) probe or any other probe, includes aresistive heating element, such as a heater wire, routed through theprobe. Over time, as the heater wire ages, the heater wire may degrade,causing characteristics of the heater wire to change. These changingcharacteristics may be monitored and plotted over time, for example. Theremaining useful life of the probe may then be determined and reportedbased upon the monitored characteristics.

FIG. 1 is a diagram illustrating aircraft 10 that includes a pluralityof probes 12 a-12 n. While illustrated as a commercial aircraft, othervehicles, such as unmanned aerial vehicles, helicopters and groundvehicles may also include probes 12 a-12 n configured to sensecharacteristics of the environment. Probes 12 a-12 n may be any type ofprobe such as, but not limited to, pitot probes, TAT probes,angle-of-attack (AOA) probes and any other probes that may include aresistive heating element.

FIG. 2 illustrates aircraft probe 12 a that includes resistive heatingelement 14. While illustrated in FIG. 2 as a TAT probe 12 a, aircraftprobe 12 a may be any other type of probe 12 a-12 n or sensing element.Probe 12 a is connected to receive control and power from control andinterface circuit 16. Control and interface circuit 16 may beimplemented local to probe 12 a (e.g., implemented as a “smart probe”)or remote of probe 12 a. Control and interface circuit 16 may include,for example, a microcontroller, programmable logic device, applicationintegrated circuit (ASIC), or any other digital and/or analog circuitry.

Resistive heating element 14, which may be a heater wire, for example,may receive power directly, or through control and interface circuit 16,from aircraft power bus 18 to provide heating for probe 12 a. Power bus18 may be any direct current (DC) or alternating current (AC) aircraftpower bus. For example, resistive heating element 14 may receive powerfrom a 28 Volt DC power bus. An operational current, based on the powerreceived from power bus 18, flows through resistive heating element 14,which provides heating for probe 12 a. Control and interface circuit 16may also be connected to aircraft avionics 20. Alternatively, controland interface circuit 16 may be implemented integral to aircraftavionics 20. Control and interface circuit 16 may be configured toprovide data to, and receive data from, aircraft avionics 20.

Current sensors 22 a and 22 b may sense current flowing into, and outof, resistive heating element 14 at heating element input 26 a andheating element output 26 b, respectively. Current sensors 22 a and 22 bmay provide a signal indicative of the sensed current at the respectivelocations to control and interface circuit 16. Temperature sensor 24 maybe positioned to sense a temperature of probe 12 a and provide thesensed temperature to control and interface circuit 16. In otherembodiments, a temperature may be estimated, for example, based uponsensed aircraft conditions and provided to control and interface circuit16 from avionics 20. For example, avionics 20 may determine a presentaltitude and/or airspeed of aircraft 10 and may estimate the temperatureof probe 12 a based upon, among other items, the present altitude and/orairspeed. Current sensors 22 a and 22 b may be any devices that sensecurrent. In an embodiment, current sensors 22 a and 22 b may benon-contact current sensors such as, for example, a current transformeror Hall effect sensor.

Thermal imager 28 may be located integral to, or separate from, aircraft10. Thermal imager 28 may be any device capable of receiving infraredradiation, for example, and providing an electrical output indicative ofthe received infrared radiation. While not illustrated as such, thermalimager 28 may be connected to communicate with control and interfacecircuit 16 and/or avionics 20 through a wired or wireless connection.This way, thermal images of probe 12 a may be obtained, analyzed andstored over time.

Radio-frequency (RF) antenna 30 is any antenna structure capable ofemitting and/or receiving an RF signal. RF antenna 30 may be configuredto receive power and emit RF radiation that may be received by heatingelement 14, for example. Heating element 14, which may include a largeloop of heater wire having dielectric properties, may act as an antenna,capable of emitting and/or receiving RF energy. RF antenna 30 may belocated integral to, or separate from, aircraft 10. RF antenna 30 may beconnected to communicate with control and interface circuit 16 and/oravionics 20 through a wired or wireless connection (not shown). Thisway, control and interface circuit 16 may control RF antenna 30 to emita plurality of RF signal frequencies, and/or may analyze a response ofRF antenna 30 to an RF signal emitted by heater wire 14, for example.

Over time, resistive heating element 14 may degrade, and eventuallybreak down such that current may no longer flow through resistiveheating element 14 to provide heating for probe 12 a. Once resistiveheating element 14 has broken down, aircraft probe 12 a must be repairedor replaced. A TAT probe, for example, may be utilized, among otherthings, to determine a Mach number for the aircraft. The Mach number maybe needed for takeoff and thus, the TAT probe may be required to befunctional prior to taking off. If the TAT probe is malfunctioning, itmust be replaced, which may cause undesirable flight delays. If theremaining useful life of the TAT probe is known, the TAT probe can bereplaced between flights or at another convenient time for repair,preventing delays or other costs incurred due to an unexpected failureof the probe.

FIG. 3 is a diagram illustrating an embodiment of heating element 14 ofaircraft probe 12 a. Heating element 14, which is shown as a heater wirein the embodiment illustrated in FIG. 3, includes lead wire 40, heaterelement 42, insulation 44 and sheath casing 46. Sheath casing 46 may bea metallic sheath and thus, there may be a measurable capacitancebetween sheath casing 46 and lead wire 40. Ring oscillator 48 may be anyoscillator circuit in which a capacitance may be utilized to drive anoutput frequency, for example. Capacitive measurement circuit 50 is anycircuit capable of providing a value to control and interface circuit16, for example, that allows control and interface circuit 16 todetermine the capacitance between sheath casing 46 and lead wire 40. Forexample, capacitive measurement circuit 50 may be a resistor-capacitor(RC) or resistor-inductor-capacitor (RLC) circuit connected such thatthe capacitor for the RC or RLC circuit is the capacitance between leadwire 40 and metallic sheath 46. In some embodiments, ring oscillator 48and capacitive measurement circuit 50 may be the same circuit.

During operation of probe 12 a, changes may occur, for example, due tohot and cold cycles as well as other varying environmental conditionssuch as temperature, pressure, humidity, environmental gases andaerosols, among others. These environmental conditions, in addition tothe temperature cycling of probe 12 a, may cause a sealing of heatingelement 14 to become compromised, leading to foreign material leakinginto insulation 44 and heater element 42. Heating element 14 may beoxidized and the dielectric material properties of heating element 14may change. This may lead to certain characteristics of heating element14 such as resistance and the capacitance between wire 40 and metallicsheath 46, among other characteristics, to change over time.

FIG. 4 illustrates functions 60 of a monitored characteristic over thelife of several resistive heating elements 14 of several respectiveprobes 12 a-12 n until breakdown 62. A heater wire, for example, maydegrade as the heater wire ages. This degradation may cause changes incharacteristics of heater wire 14 such as resistance and capacitance.For example, the current drawn through resistive heating element 14 mayincrease monotonically over the life of heating element 14. Thisincrease is not linear, but rather follows exponential function 60, witha point 64 of interest. Point 64 may be the point on function 60, forexample, at which the slope transitions from greater than 1 to lessthan 1. While the maximum normalized characteristic and life of eachresistive heating element 14 at breakdown 62 may fluctuate, exponentialfunction 60 of the heater characteristic over the life of each resistiveheating element 14 remains similar.

Functions 60 shown in FIG. 4 may be obtained, for example, throughtesting of probes 12 a-12 n. For a selected characteristic of heatingelement 14, such as current draw, capacitance, leakage current, thermalresponse, or other characteristic, function 60 may be determined throughthe testing of probes 12 a-12 n. Current may be cycled, for example,until heating element 14 of a respective test probe 12 a-12 n breaksdown. A characteristic of heating element 14 may be monitored each testcycle and plotted over time until breakdown 62. The plots of thecharacteristic may then be utilized to determine function 60. Function60 may then be utilized during normal operation of a probe 12 a-12 n todetermine, for example, a half-life estimate of heating element 14. Inother embodiments, other algorithms may be utilized to determine theremaining useful life of heating element 14 based upon the plotted data.

In an example embodiment, current may be sampled using current sensor 22a or 22 b at any time during flight and provided to control andinterface circuit 16 several times each flight. This sampled current maybe stored in a memory of control and interface circuit 16, for example,or some other memory or storage device. Because current is directlyaffected by temperature, a temperature may also be sensed usingtemperature sensor 24 and stored along with a respective sensed currentso that the current may be normalized with respect to temperature.Alternatively, a temperature may be estimated using data from avionics20, for example. Control and interface circuit 16 may utilize the sensedor estimated temperature to directly normalize the current prior tostorage, or may store the two values for later normalization.

Function 60, determined during testing, may be used in conjunction withthe stored normalized current to determine the half-life estimate ofheating element 14. For example, the stored normalized current may beplotted by control and interface circuit 16 such that a present slope ofthe plot may be determined. This present slope may be utilized, forexample, to detect point 64 by detecting that the slope of the plot hastransitioned from greater than 1 to less than 1, or any other point onfunction 60. Once point 64 is detected, for example, the half-life ofheating element 14 may be determined.

FIGS. 5A and 5B are charts illustrating monitored current based upon anormal operating voltage and a low voltage for heating element 14 overtime. FIG. 5A illustrates both a current draw 70 for an operatingvoltage and current draw 72 for a low voltage. FIG. 5B shows a zoomed inview of current draw 72 for a low voltage. In an embodiment, theoperating voltage may be, for example, 28 Volts while the low voltagemay be, for example, 0.1 Volts.

At the end of the life of heating element 14, thermal fatigue causesheater wire 40 to fracture and gradually become electrically open, whichincreases the resistance of heating element 14. At low voltage, electrontunneling is the main mechanism for electrical current conduction.Arcing that occurs at 0.1 Volts may indicate a micro fracture with a gapon the order of 1 nanometer (nm), while arcing that occurs at 28 Voltsmay correspond to an approximately 0.3 micrometer (um) gap. Thus, microfractures may be detected by monitoring a current response using a lowvoltage prior to failure of heating element 14.

As seen in FIG. 5A, the current drawn during normal operation (e.g.,receiving 28 Volts from power bus 18) remains substantially greater thanzero over all cycles. As seen in FIG. 5B, the current drawn with 0.1Volts begins to degrade to, and remain at, zero while the current drawnduring normal operation continues to be substantially greater than zero.This degradation of current to zero at low voltage indicates that amicro fracture is present in heating element 14. This micro fracture mayeventually grow in size, ultimately causing failure of probe 12 a. Bydetecting these micro fractures early, failure of probe 12 a may bepredicted such that a remaining useful life of heating element 14, andin turn, probe 12 a, may be determined.

The amount of remaining useful life of probe 12 a following detection ofa micro fracture may be determined, for example, through testing ofprobes 12 a-12 n. Current may be cycled, for example, until heatingelement 14 of a respective test probe 12 a-12 n breaks down. Betweentest cycles, a low voltage may be provided to test probes 12 a-12 n.While the low voltage is supplied, a current may be sensed and stored.This way, the cycle at which a micro fracture on the order of 1 nm isdetected may be determined. Following detection of the micro fracture,once the respective test probe 12 a-12 n breaks down, a percentage oflife after detection of the micro fracture may be determined.

During normal operation of probes 12 a-12 n, a low voltage may beprovided, for example, by control and interface circuit 16 betweenflights of aircraft 10, or at any other time during which heatingelement 14 is not receiving an operational voltage. Current sensor 22 aor 22 b may be utilized by control and interface circuit 16 to obtain asensed current while providing the low voltage. Upon detection of thesensed current going to zero at low voltage, a remaining useful life maybe determined based upon the percentage of life determined duringtesting of the probes 12 a-12 n.

FIG. 6 is a chart illustrating a resonant frequency over time forheating element 14 of aircraft probe 12 a. FIG. 6 also illustrates acurve 80 fit to the resonant frequency plot over time. As seen in FIG.3, a capacitance exists between metallic sheath 46 and lead wire 40.While in operation, changes to heating element 14 happen due to hot andcold cycles in addition to varying environmental conditions such astemperature, pressure, humidity, environmental gases, and aerosols,among others. These conditions, in addition to the temperature cyclingof probe 12 a, cause the probe heater sealing to be compromised, whichmay lead to various materials leaking into insulation 44 and heaterelement 42. Wire 40 may be oxidized and dielectric material propertiesmay change, leading to a change in capacitance and resistance of heatingelement 14. Because the capacitance changes, the resonant frequency alsochanges.

In an embodiment, to obtain the resonant frequency, the capacitancebetween metallic sheath 46 and lead wire 40 may be swept withfrequencies ranging from 1 kilohertz (KHz) to 100 megahertz (MHz) bycontrol and interface circuit 16, for example, through capacitivemeasurement circuit 50. The peak frequency response and bandwidth may beidentified by control and interface circuit 16. This peak frequencyresponse may be monitored and stored over time and plotted as shown inFIG. 6. In one embodiment, a function 80 obtained during testing ofprobes 12 a-12 n, for example, may then be utilized in conjunction withthe stored and plotted resonant frequency to determine a remaininguseful life of probe 12 a. For example, it may be determined when theslope of the plotted resonance transitions from greater than −1 to lessthan −1, or when the slope transitions from a negative value to apositive value, as seen in FIG. 6. Determination of a present point onfunction 80 allows control and interface circuit 16 to determine aremaining useful life of probe 12 a.

In another embodiment, algorithms that separate data indicative of ahealthy probe and data indicative of an increasingly unhealthy probe maybe executed utilizing various signal processing techniques to determinethe remaining useful life of heating element 14. For example,time-frequency analysis, machine learning, index theory and other signalprocessing techniques may be utilized to determine remaining useful lifeof heating element 14 based upon the monitored resonant frequency.

In another embodiment, ring oscillator 48 may be utilized with probe 12a as the driving element of the output frequency of ring oscillator 48.Ring oscillator 48, or other oscillator circuit, has an output frequencydependent upon the structure of the circuit and the input voltage to thecircuit. By controlling the input voltage, the output frequency of ringoscillator 48 can be made dependent solely upon the capacitance betweenmetallic sheath 46 and lead wire 40. As this capacitance changes asheating element 14 degrades, so does the output frequency of ringoscillator 48. The changing output frequency may be stored and plottedover time to determine a remaining useful life of probe 12 a. Forexample, testing of probes 12 a-12 n may result in a function of theoutput frequency similar to that shown in FIG. 4 or 6. This function, inconjunction with the stored and plotted output frequency, may beutilized to determine the remaining useful life of probe 12 a duringnormal operation.

In another embodiment, RF antenna 30 may be controlled to sweep a widerange of frequencies. For example, control and interface circuit 16 mayprovide AC power to antenna 30 at varying frequencies to facilitateemission of RF radiation at a plurality of frequencies from antenna 30.The S12 parameter for heating element 14, which is a measure of thepower received at heating element 14 from the RF emission of antenna 30,may be determined and monitored by control and interface circuit 16.

This antenna response of heating element 14 may be analyzed by controland interface circuit 16 to determine, for example, a resonant frequencyresponse of heating element 14. The antenna properties of heatingelement 14 may be dependent upon, among other things, length and shapeof heating element 14, and dielectric properties of heating element 14.These dielectric properties may include, for example, permittivity,permeability, homogeneity and thickness, among others. As the wire agesand degrades, the dielectric properties of heating element 14 maychange, which may lead to a change in the resonant frequency of heatingelement 14.

The determined resonant frequency may be monitored and plotted over timeby control and interface circuit 16. Testing of probes 12 a-12 n may beutilized to determine a function at which the resonant frequency changesover time. This function may substantially follow those shown in FIG. 4or 6, for example. This function may then be utilized to determine, forexample, a half-life of heating element 14 base upon the determinedresonant frequency during normal operation of probe 12 a. In otherembodiments, an algorithm may be executed using signal processingtechniques to determine the remaining useful life of probe 12 a basedupon the resonant frequency changes over time.

In another embodiment, control and interface circuit 16 may provide ACpower to heating element 14 at varying frequencies to facilitateemission of RF radiation at a plurality of frequencies from heatingelement 14. Control and interface circuit 16 may then monitor the S12parameter of RF antenna 30 to determine a resonant frequency, which maybe monitored and analyzed over time to determine a half-life of heatingelement 14.

FIGS. 7A and 7B are representatives of thermal images of aircraft probe12 a. These images may be obtained, for example, using thermal imager 28(FIG. 2). FIG. 7A illustrates probe 12 a at a beginning of its lifetime,while FIG. 7B illustrates probe 12 a at a later point in its lifetime.The thermal images may be utilized, for example, to determine a thermalresponse of heating element 14 over time. Points of interest 90 a and 92a show a thermal response of heating element 14 at the beginning of lifeof heating element 14, while points of interest 90 b and 92 b show athermal response of heating element 14 at a later point in life. Whileillustrated as images captured at various view angles with respect toprobe 12 a, any number of thermal images at any angle with respect toprobe 12 a may be obtained by imager 28. Other points of interest forheating element 14 may also be monitored separately, or in conjunctionwith, points 90 a, 90 b, 92 a and 92 b to determine the thermal responseof heating element 14.

As seen in FIG. 7B, the temperature at points 90 b and 92 b of heatingelement 14 has increased from the temperature at points 90 a and 92 ashown in FIG. 7A. This increase may be due to changes in heating elementcaused by degradation of heating element 14 over time. The increase intemperature over time may substantially follow an exponential functionsimilar to those illustrated in FIG. 4. Because of this, the thermalresponse of heating element 14 (e.g., the increase in temperature seenat points 90 b and 92 b) may be stored and plotted over time duringnormal operation of probe 12 a. An exponential function determinedduring testing of probes 12 a-12 n, for example, may then be used inconjunction with the stored and plotted thermal response to determine ahalf-life of heating element 14.

FIG. 8 is a flowchart illustrating method 100 of determining a remaininguseful life of probe 12 a based upon detected micro-fractures. At step102, a low voltage is provided to heating element 14 of probe 12 a. Thislow voltage may be provided at any time, such as right after power downof probe 12 a. The low voltage may be approximately 0.1 V, which may below enough to detect micro fractures on the order of 1 nm.

A step 104, while the low voltage is being supplied to heating element14, current is sensed by one of current sensors 22 a and 22 b. Thesensed current may be provided to, and stored by, control and interfacecircuit 16. The process in steps 102 and 104 is repeated over time, andthe sensed current is monitored by control and interface circuit 16 atstep 106. At step 108, micro fractures in heating element 14 aredetected based upon the monitored current. For example, if the monitoredcurrent has dropped to zero, as illustrated in FIG. 5B, a micro fractureis detected by control and interface circuit 16.

Following detection of an approximately 1 nm micro fracture, theremaining useful life of heating element 14 may be estimated. Thisestimation may be based upon testing of heating elements 14, forexample. Probes 12 a-12 n may be tested to determine an averageremaining useful life of heating element 14 following detection of amicro fracture of approximately 1 nm.

Once the remaining useful life of resistive heating element 14 isdetermined by control and interface circuit 16, the remaining usefullife may be reported at step 110. This report may be made to the cockpitthrough avionics 20, or to some other computer system. By reporting theremaining useful life, probes 12 a-12 n may be replaced prior tobreaking down and thus, unnecessary flight delays may be avoided.

FIG. 9 is a flowchart illustrating method 120 of determining a remaininguseful life of a probe 12 a-12 n based upon monitored current drawn overtime by a respective heating element 14. At step 122, an exponentialfunction for a respective probe 12 a-12 n may be determined, forexample, through testing of similar probes 12 a-12 n. The test probesmay have a test current provided to a respective resistive heatingelement 14 during a plurality of test cycles. The current may be sensedat common points during each of the plurality of test cycles and thesensed currents may then be plotted and functions 60 may be determinedas shown in FIG. 4 for the respective resistive heating elements 14.This exponential function may then be used for all similar probes 12a-12 n during normal probe operation in order to determine a half-lifeof a respective resistive heating element 14, for example.

During normal operation of probes 12 a-12 n, current through resistiveheating element 14 may fluctuate based upon the operating point of probe12 a-12 n. Therefore, it may be desirable at step 124 to sense theoperational current at a similar point of operation of heating element14. For example, upon initial power-on of resistive heating element 14,the current may rise to a peak current. As resistive heating element 14increases in temperature, the current through resistive heating element14 will decrease to a lower, steady-state current. Thus, current may besampled and stored consistently at the peak value, at the steady-statevalue, or at some other expected value, for example.

Temperature may also be sensed and provided to control and interfacecircuit 16 using temperature sensor 24. At step 126, control andinterface circuit 16 may normalize the sensed current using the sensedtemperature. Steps 124 and 126 may be repeated and the normalized sensedcurrent may be plotted over several flights, for example, to establish acurve fit. The curve fit may substantially follow the exponentialfunction determined, for example, at step 122. At step 128, the presentslope of the sensed current may be determined based upon the exponentialfunction. This slope may then be utilized to determine the half-life ofresistive heating element 14. By knowing the half-life of resistiveheating element 14, the remaining useful life of resistive heatingelement 14 may be determined by control and interface circuit 16, forexample.

Once the remaining useful life of resistive heating element 14 isdetermined by control and interface circuit 16, the remaining usefullife may be reported at step 130. This report may be made to the cockpitthrough avionics 20, or to some other computer system. By reporting theremaining useful life, probes 12 a-12 n may be replaced prior tobreaking down and thus, unnecessary flight delays may be avoided.

FIG. 10 is a flowchart illustrating method 140 of determining aremaining useful life of probes 12 a-12 n based upon a determinedcapacitance of a respective heating element 14. At step 142, anexponential function for a respective probe 12 a-12 n may be determined,for example, through testing of similar test probes. The test probes mayhave a test current provided to a respective resistive heating element14 during a plurality of test cycles to simulate the life of therespective probe 12 a-12 n. The current may be cycled until failure ofeach respective probe 12 a-12 n. The capacitance of each respectiveheating element 14 may be determined during each cycle, for example. Anexponential function, such as those shown in FIG. 4, may be determinedbased upon the determined capacitances. This exponential function maythen be used for all similar probes 12 a-12 n during normal probeoperation in order to determine a half-life of a respective resistiveheating element 14, for example.

At step 144, during normal operation of probes 12 a-12 n, capacitivemeasurement circuit 50 may be utilized to determine a capacitancebetween metallic sheath 46 and lead wire 40. The capacitance andcapacitance measurement circuit 50 may be directly affected bytemperature. At step 146, temperature may be sensed and provided tocontrol and interface circuit 16 using temperature sensor 24. Steps 144and 146 may be repeated throughout the life of probe 12 a and controland interface circuit 16 may normalize the determined capacitance usingthe sensed temperature and may plot the normalized determinedcapacitance over several flights, for example, to establish a curve fit.The curve fit may substantially follow the exponential functiondetermined, for example, at step 142. At step 148, the present slope ofthe determined capacitance may be determined based upon the exponentialfunction. This slope may then be utilized to determine the half-life ofresistive heating element 14. By knowing the half-life of resistiveheating element 14, the remaining useful life of resistive heatingelement 14 may be determined by control and interface circuit 16, forexample.

Once the remaining useful life of resistive heating element 14 isdetermined by control and interface circuit 16, the remaining usefullife may be reported at step 150. This report may be made to the cockpitthrough avionics 20, or to some other computer system. By reporting theremaining useful life, probes 12 a-12 n may be replaced prior tobreaking down and thus, unnecessary flight delays may be avoided.

FIG. 11 is a flowchart illustrating method 160 of determining aremaining useful life of probes 12 a-12 n based upon a measured leakagecurrent for a respective heating element 14. At step 162, an exponentialfunction for a respective probe 12 a-12 n may be determined, forexample, through testing of similar probes 12 a-12 n. The test probesmay have a test current provided to a respective resistive heatingelement 14 during a plurality of test cycles to simulate the life of therespective probe 12 a-12 n. Current may be sampled by both currentsensors 22 a and 22 b and provided to control and interface circuit 16.The current from current sensor 22 b for example, may be subtracted fromthe current from current sensor 22 a to determine a leakage current ofresistive heating element 14. The current may be cycled until failure ofeach respective probe 12 a-12 n at which time the exponential function,similar to those illustrated in FIG. 4, may be determined.

At step 164, during normal operation of probes 12 a-12 n, leakagecurrent from resistive heating element 14 may be determined based uponsensed current from both current sensors 22 a and 22 b. Current throughresistive heating element 14 may fluctuate based upon the operatingpoint of probe 12 a-12 n. Therefore, it may be desirable to determinethe leakage current at a similar point of operation of heating element14 each time the current is sensed and stored. For example, upon initialpower-on of resistive heating element 14, the current may rise to a peakcurrent. As resistive heating element 14 increases in temperature, thecurrent through resistive heating element 14 will decrease to a lower,steady-state current. Thus, current may be sampled and storedconsistently at the peak value, at the steady-state value, or at someother expected value, for example.

Temperature may also be sensed and provided to control and interfacecircuit 16 using temperature sensor 24. At step 166, control andinterface circuit 16 may normalize the determined leakage current usingthe sensed temperature. Steps 164 and 166 may be repeated and controland interface circuit 16 may plot the normalized leakage current overseveral flights, for example, to establish a curve fit. The curve fitmay substantially follow the exponential function determined, forexample, at step 162. At step 168, the present slope of the determinedleakage current may be determined based upon the exponential function.This slope may then be utilized to determine the half-life of resistiveheating element 14. By knowing the half-life of resistive heatingelement 14, the remaining useful life of resistive heating element 14may be determined by control and interface circuit 16, for example.

Once the remaining useful life of resistive heating element 14 isdetermined by control and interface circuit 16, the remaining usefullife may be reported at step 170. This report may be made to the cockpitthrough avionics 20, or to some other computer system. By reporting theremaining useful life, probes 12 a-12 n may be replaced prior tobreaking down and thus, unnecessary flight delays may be avoided.

FIG. 12 is a flowchart illustrating method 180 of determining aremaining useful life of probes 12 a-12 n based upon a resonantfrequency of a respective heating element 14. At step 182, anexponential function for a respective probe 12 a-12 n may be determined,for example, through testing of similar test probes. The exponentialfunction may be similar to that shown in FIG. 6. The test probes mayhave a test current provided to a respective resistive heating element14 during a plurality of test cycles to simulate the life of therespective probe 12 a-12 n. The current may be cycled until failure ofeach respective probe 12 a-12 n. The resonant frequency based on thecapacitance of each respective heating element 14 may be determinedduring each cycle, for example, using capacitive measurement circuit 50.Alternatively, an output frequency of oscillator circuit 48 driven bythe capacitance of heating element 14 may be determined during eachcycle. An exponential function, such as that shown in FIG. 4 or FIG. 6,may be determined based upon the determined resonant or outputfrequency. This exponential function may then be used for all similarprobes 12 a-12 n during normal probe operation in order to determine ahalf-life of a respective resistive heating element 14, for example.

At step 184, during normal operation of probes 12 a-12 n, capacitivemeasurement circuit 50 may be used to determine a resonant frequency ofheating element 14 or ring oscillator circuit 48 may be utilized todetermine an output frequency of ring oscillator 48 driven by thecapacitance of heating element 14. At step 186, temperature may also besensed and provided to control and interface circuit 16 usingtemperature sensor 24. Control and interface circuit 16 may normalizethe determined resonant frequency using the sensed temperature. Steps184 and 186 may be repeated and control and interface circuit 16 mayplot the normalized resonant or output frequency over several flights,for example, to establish a curve fit. While theoretical capacitance isnot directly affected by temperature, in practice both the capacitanceand ring oscillator circuit 48 may be directly affected by temperature.The curve fit may substantially follow the exponential functiondetermined, for example, at step 182. At step 188, the present slope ofthe determined resonance may be determined based upon the exponentialfunction. This slope may then be utilized to determine the half-life ofresistive heating element 14. By knowing the half-life of resistiveheating element 14, the remaining useful life of resistive heatingelement 14 may be determined by control and interface circuit 16, forexample.

Once the remaining useful life of resistive heating element 14 isdetermined by control and interface circuit 16, the remaining usefullife may be reported at step 190. This report may be made to the cockpitthrough avionics 20, or to some other computer system. By reporting theremaining useful life, probes 12 a-12 n may be replaced prior tobreaking down and thus, unnecessary flight delays may be avoided.

FIG. 13 is a flowchart illustrating method 200 of determining aremaining useful life of probes 12 a-12 n based upon thermal imaging ofa respective heating element 14. At step 202, an exponential functionfor a respective probe 12 a-12 n may be determined, for example, throughtesting of similar test probes. The exponential function may be similarto that shown in FIG. 4. The test probes may have a test currentprovided to a respective resistive heating element 14 during a pluralityof test cycles to simulate the life of the respective probe 12 a-12 n.The current may be cycled until failure of each respective probe 12 a-12n. Thermal images may be taken by thermal imager 28 for each respectiveheating element 14 each cycle, for example. An exponential function,such as that shown in FIG. 4, may be determined based upon the thermalimages. This exponential function may then be used for all similarprobes 12 a-12 n during normal probe operation in order to determine ahalf-life of a respective resistive heating element 14, for example.

At step 204, during normal operation of probes 12 a-12 n, thermal imager28 may be utilized to obtain thermal images of heating element 14. Atstep 206, temperature may also be sensed and provided to control andinterface circuit 16 using temperature sensor 24. Control and interfacecircuit 16 may normalize the thermal data using the sensed temperature.Control and interface circuit 16 may also normalize the thermal imagedata to compensate for probe shape and surface emissivity, for example.Steps 204 and 206 may be repeated and control and interface circuit 16may plot the normalized thermal data over several flights, for example,to establish a curve fit. The curve fit may substantially follow theexponential function determined, for example, at step 202. At step 208,the present slope of the determined thermal data be determined basedupon the exponential function. This slope may then be utilized todetermine the half-life of resistive heating element 14. By knowing thehalf-life of resistive heating element 14, the remaining useful life ofresistive heating element 14 may be determined by control and interfacecircuit 16, for example.

Once the remaining useful life of resistive heating element 14 isdetermined by control and interface circuit 16, the remaining usefullife may be reported at step 210. This report may be made to the cockpitthrough avionics 20, or to some other computer system. By reporting theremaining useful life, probes 12 a-12 n may be replaced prior tobreaking down and thus, unnecessary flight delays may be avoided.

FIG. 14 is a flowchart illustrating method 220 of determining aremaining useful life of probes 12 a-12 n based upon an antenna responseof a respective heating element 14. At step 222, an exponential functionfor a respective probe 12 a-12 n may be determined, for example, throughtesting of similar test probes. The exponential function may be similarto that shown in FIG. 4. The test probes may have a test currentprovided to a respective resistive heating element 14 during a pluralityof test cycles to simulate the life of the respective probe 12 a-12 n.The current may be cycled until failure of each respective probe 12 a-12n. RF antenna 30 may be utilized during each cycle, for example, tosweep frequencies of RF radiation to heating element 14. An exponentialfunction, such as that shown in FIG. 4, may be determined based upon adetected resonant frequency of heater element 14 in response to the RFradiation from antenna 30 over each cycle. This exponential function maythen be used for all similar probes 12 a-12 n during normal probeoperation in order to determine a half-life of a respective resistiveheating element 14, for example.

At step 224, during normal operation of probes 12 a-12 n, RF antenna 30may be utilized to provide RF radiation to heating element 14. At step226, control and interface circuit 16 may monitor the S12 parameter ofheating element 14 to determine a resonant frequency response of heatingelement 14 to the RF radiation. In an alternative embodiment, heatingelement 14 may be energized to provide RF radiation to antenna 30 andcontrol and interface circuit 16 may monitor the S12 parameter ofantenna 30 to determine a resonant frequency response of antenna 30.

Steps 224 and 226 may be repeated and control and interface circuit 16may plot the resonant frequency response over several flights, forexample, to establish a curve fit. The curve fit may substantiallyfollow the exponential function determined, for example, at step 222. Atstep 228, the present slope of the determined RF response may bedetermined based upon the exponential function. This slope may then beutilized to determine the half-life of resistive heating element 14. Byknowing the half-life of resistive heating element 14, the remaininguseful life of resistive heating element 14 may be determined by controland interface circuit 16, for example. During steps 224 and 226, the RFresponse may be normalized to account for, among other things,temperature. This may be accomplished using data from temperature sensor24, or an estimate temperature based upon data from avionics 20, forexample.

Once the remaining useful life of resistive heating element 14 isdetermined by control and interface circuit 16, the remaining usefullife may be reported at step 230. This report may be made to the cockpitthrough avionics 20, or to some other computer system. By reporting theremaining useful life, probes 12 a-12 n may be replaced prior tobreaking down and thus, unnecessary flight delays may be avoided.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

A system for an aircraft includes a probe and a control circuit. Theprobe includes a heater that includes a resistive heating element routedthrough the probe. An operational voltage is provided to the resistiveheating element to provide heating for the probe. The control circuit isconfigured to provide the operational voltage and monitor a capacitancebetween the resistive heating element and a metallic sheath of theheater over time. The control circuit is further configured to determinea remaining useful life of the probe based on the capacitance.

The system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

A further embodiment of the foregoing system, wherein the controlcircuit is further configured to monitor the capacitance over time bystoring the capacitance over a plurality of flights of the aircraft.

A further embodiment of any of the foregoing systems, wherein thecontrol circuit is configured to store and plot the capacitance over theplurality of flights of the aircraft and determine a present slope of anexponential function of the plot of the capacitance.

A further embodiment of any of the foregoing systems, wherein thecontrol circuit is further configured to determine a half-life estimateof the resistive heating element based upon the present slope of theexponential function.

A further embodiment of any of the foregoing systems, wherein thecontrol circuit is further configured to normalize the capacitance basedupon a temperature of the probe.

A further embodiment of any of the foregoing systems, further comprisinga temperature sensor configured to sense a temperature of the probe andprovide the sensed temperature to the control circuit, wherein thecontrol circuit is configured to normalize the capacitance based uponthe sensed temperature.

A further embodiment of any of the foregoing systems, wherein thetemperature of the probe is an estimated temperature.

A method for determining a remaining useful life of a probe thatincludes a heater, the method includes providing an operational voltageto a resistive heating element of the aircraft probe during a pluralityof flights of an aircraft to provide heat for the probe; determining, bya control circuit, a capacitance between a resistive heating element ofthe heater and a metallic sheath of the heater during the plurality offlights; and determining the remaining useful life of the probe basedupon the determined capacitance over time.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

A further embodiment of the foregoing method, further including testinga plurality of test probes over an entire life of the test probes todetermine an exponential function; and determining a half-life of theresistive heating element based upon the exponential function and thedetermined capacitance over time.

A further embodiment of any of the foregoing methods, whereindetermining, by the control circuit, the capacitance between theresistive heating element of the heater and the metallic sheath of theheater during the plurality of flights includes normalizing thedetermined capacitance based on a temperature of the probe.

A probe system includes a heater and a control circuit. The heaterincludes a resistive heating element routed through the probe system. Anoperational voltage is provided to the resistive heating element toprovide heating for the probe system. The control circuit is configuredto provide the operational voltage and monitor a capacitance between theresistive heating element and a metallic sheath of the heater over time.The control circuit is further configured to determine a remaininguseful life of the probe system based on the capacitance.

The probe system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components:

A further embodiment of the foregoing probe system, wherein the controlcircuit is further configured to monitor the capacitance over time bystoring the capacitance over a plurality of flights of the aircraft.

A further embodiment of any of the foregoing probe systems, wherein thecontrol circuit is configured to store and plot the capacitance over theplurality of flights of the aircraft and determine a present slope of anexponential function of the plot of the capacitance.

A further embodiment of any of the foregoing probe systems, wherein thecontrol circuit is further configured to determine a half-life estimateof the resistive heating element based upon the present slope of theexponential function.

A further embodiment of any of the foregoing probe systems, wherein thecontrol circuit is further configured to normalize the capacitance basedupon a temperature of the probe system.

A further embodiment of any of the foregoing probe systems, furthercomprising a temperature sensor configured to sense a temperature of theprobe system and provide the sensed temperature to the control circuit,wherein the control circuit is configured to normalize the capacitancebased upon the sensed temperature.

A further embodiment of any of the foregoing probe systems, wherein thetemperature of the probe is an estimated temperature.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A system for an aircraft, the system comprising: a probe thatincludes a heater comprising a resistive heating element routed throughthe probe, wherein an operational voltage is provided to the resistiveheating element to provide heating for the probe; and a control circuitconfigured to provide the operational voltage and monitor a capacitancebetween the resistive heating element and a metallic sheath of theheater over time; wherein the control circuit is further configured todetermine a remaining useful life of the probe based on the capacitance.2. The system of claim 1, wherein the control circuit is furtherconfigured to monitor the capacitance over time by storing thecapacitance over a plurality of flights of the aircraft.
 3. The systemof claim 2, wherein the control circuit is configured to store and plotthe capacitance over the plurality of flights of the aircraft anddetermine a present slope of an exponential function of the plot of thecapacitance.
 4. The system of claim 3, wherein the control circuit isfurther configured to determine a half-life estimate of the resistiveheating element based upon the present slope of the exponentialfunction.
 5. The system of claim 1, wherein the control circuit isfurther configured to normalize the capacitance based upon a temperatureof the probe.
 6. The system of claim 5, further comprising a temperaturesensor configured to sense a temperature of the probe and provide thesensed temperature to the control circuit, wherein the control circuitis configured to normalize the capacitance based upon the sensedtemperature.
 7. The system of claim 5, wherein the temperature of theprobe is an estimated temperature.
 8. A method for determining aremaining useful life of a probe that includes a heater, the methodcomprising: providing an operational voltage to a resistive heatingelement of the aircraft probe during a plurality of flights of anaircraft to provide heat for the probe; determining, by a controlcircuit, a capacitance between a resistive heating element of the heaterand a metallic sheath of the heater during the plurality of flights; anddetermining the remaining useful life of the probe based upon thedetermined capacitance over time.
 9. The method of claim 8, furthercomprising: testing a plurality of test probes over an entire life ofthe test probes to determine an exponential function; and determining ahalf-life of the resistive heating element based upon the exponentialfunction and the determined capacitance over time.
 10. The method ofclaim 8, wherein determining, by the control circuit, the capacitancebetween the resistive heating element of the heater and the metallicsheath of the heater during the plurality of flights comprisesnormalizing the determined capacitance based on a temperature of theprobe.
 11. A probe system comprising: a heater comprising a resistiveheating element routed through the probe system, wherein an operationalvoltage is provided to the resistive heating element to provide heatingfor the probe system; and a control circuit configured to provide theoperational voltage and monitor a capacitance between the resistiveheating element and a metallic sheath of the heater over time; whereinthe control circuit is further configured to determine a remaininguseful life of the probe system based on the capacitance.
 12. The probesystem of claim 11, wherein the control circuit is further configured tomonitor the capacitance over time by storing the capacitance over aplurality of flights of the aircraft.
 13. The probe system of claim 12,wherein the control circuit is configured to store and plot thecapacitance over the plurality of flights of the aircraft and determinea present slope of an exponential function of the plot of thecapacitance.
 14. The probe system of claim 13, wherein the controlcircuit is further configured to determine a half-life estimate of theresistive heating element based upon the present slope of theexponential function.
 15. The probe system of claim 11, wherein thecontrol circuit is further configured to normalize the capacitance basedupon a temperature of the probe system.
 16. The probe system of claim15, further comprising a temperature sensor configured to sense atemperature of the probe system and provide the sensed temperature tothe control circuit, wherein the control circuit is configured tonormalize the capacitance based upon the sensed temperature.
 17. Theprobe system of claim 15, wherein the temperature of the probe is anestimated temperature.